Many of the advances made in the electronics industry have occurred due to increasing miniaturization of circuits and chip components. This has been especially true in the computer industry, where increasingly smaller processor components have allowed additional speed and portability to occur. In order to package integrated circuits (IC), miniature wires must be soldered to the electrodes of a chip. The most widely used process for making these connections has become known as a “tap bonding system” (TAB).
Most chips are designed to contain an array of electrodes that terminate on an exterior portion of the chip, and show up as miniature pads or posts. Each of these pads is coated with a thin layer of solder alloy, and must be individually connected to a lead wire that integrates the circuit. While the electrodes must be each individually connected to specific lead wires, the process of making the connections is typically accomplished in a bonding single step. Using the well known TAB process, an array of electrodes and lead wires can become bonded in a single step.
A typical TAB process is generally shown in FIG. 1. A plurality of microchips 10, having electrodes 15, that are typically made of gold and spaced about 100 microns apart, are positioned on a flat surface 12, moving in a fixed direction 17 in a configuration that allows the electrodes to become aligned with one or more lead wires 20. Each electrode is coated with a thin layer of a solder (e.g. tin). The lead wire is fed by carrier tape, or film, 25 over the chips. The bonding of the electrodes to the lead wire is achieved by pressing the lead wire on top of each of the electrodes by a compression bonding tool 30. The bonding tool is heated to a designated temperature, typically 500-600° C. While the lead wire and electrodes are pressed together, heat flows from the bonding tool to the wire and it causes the solder on top of the electrode to melt so the lead wire becomes bonded to the electrode almost instantaneously.
Referring again to FIG. 1, the bonding tool 30 used in most TAB processes typically includes two portions, a head 35 and a tail 40. The head has a body 45 and a pressing face 50. The body often contains a heater (not shown) which can be heated by passing electricity through a resistor as is known in the art. The pressing face is typically made of diamond, in particular, a CVD formed diamond film. The tail is used to attach to the bonding tool to other machinery which is capable of moving the bonding tool in at least an up and down motion in order to achieve the pressing action responsible for bonding the electrodes to the lead wire.
In use, the bonding tool 30 is lowered and pressed against the carrier tape 25 which feeds the lead wires 20 over the electrodes 15. Thus the lead wires become pressed against the electrodes, and the heat transferred from the pressing face 50 of the bonding tool melts the solder on the electrodes and forms a bond with the lead wires. As soon as the bonding is completed, the bonding tool is raised, so that the carrier tape will advance with the chip attached underneath. The bonding tool is now ready to perform another pressing operation.
A number of characteristics dictate the quality of bond attained by the bonding tool. Such characteristics include, the effectiveness of heat transfer from the head to the pressing face, the flatness and reactivity of the pressing face, the distribution of heat throughout the pressing face, and the quality of attachment between the tail and the machine holding the bonding tool.
One issue facing the attachment of the tail to a machine is the transfer of heat from the head of the bonding tool through the tail, and into the machine holding the tool. Because such a transfer wastes heat energy, and because heating of the machine holding the tool is typically undesirable, the handle of traditional bonding tools has been made from a material having low thermal conductivity (i.e. heat insulating), such as Kovar, Invar, or iron-cobalt-nickel alloys.
In order to address the issues of pressing face flatness, hardness, reactivity, and heat distribution, diamond has become popular for its desirable properties in each of these areas. Particularly, CVD formed diamond has received great interested because of its cost effectiveness and ability to be deposited over a large surface area.
Despite its popularity for use in making bonding tool pressing faces, CVD formed diamond contains a number of disadvantages. For example, due to bonding and thermal expansion issues CVD diamond, may only be deposited onto a selected group of materials, such as refractory elements that react with diamond to form carbide, or certain ceramic materials that have similar thermal expansion values. Other issues also continue to persist. For example, the sticking of solder and other materials to the pressing face during repeated use of a tool continues to be problematic. The fouling of the pressing face with such materials is thought to be due at least in part, to the CVD process by which the diamond is made. While diamond itself is extremely inert, the CVD process of making diamond leaves unbound electrons on many of the carbon atoms at the pressing surface. This is due to the termination of the deposition process upon completion of a diamond mass. These unbound electrons or “dangling bonds” have a significant propensity to react with other materials with which they come in contact. Therefore, the pressing surface may become fouled over time and the useful life thereof is significantly decreased.
In addition to the ongoing issues with CVD diamond, other issues persist with current bonding tools as a whole. For example, despite the use of materials having a low thermal conductivity, heat loss and subsequent warming of the machine holding the bonding tool remains. This is due in part to insufficient or sub-optimal transfer of heat through the head and to the pressing face. Moreover, due to the heat loss in the tail, maintenance of the operating temperature at the pressing face is difficult. Additionally, most of the materials currently used to support the CVD diamond pressing face, are insufficiently rigid to adequately support the diamond over a large number of pressings. Over time, this lack of rigidity allows the diamond to begin flexing under pressure, and leads to chipping of the diamond around the corners of the pressing face. Lack of rigidity can also allow the diamond face cave in the middle, so that the center lead is not pressed as hard. Furthermore, thermal expansion mismatching and tool expense remain as problems
As such, bonding tools that provide improved performance, durability, and efficiency continue to be sought through ongoing research and development efforts.